The development of NIDDM and obesity depends on both genetic and environmental factors. To investigate the impact of the environment we compared the prevalence of NIDDM and the mean body mass index (BMI) in Mexican Pimas living in Maycoba in the Sierra Madre mountains of Mexico to those in the Pima Indians of Arizona. These two groups of Pima Indians are thought to have separated 700-1000 years ago. The Mexican Pimas of Maycoba still live a traditional lifestyle with non-mechanized agriculture and lumber milling activities. We also compared the Mexican Pimas with non-Pimas living in the same environment. The entire population of Maycoba and surrounding community, aged 20 and over, were invited to undergo a 75g oral glucose tolerance test and anthropometric measures (weight, height). NIDDM and IGT were classified using WHO criteria. 141 Pimas (70F/71M) and 193 non-Pima Mexicans (101F/92M) from Maycoba were compared to 620 Arizona Pima Indians (372F/248M) from the Gila River Indian Community. The prevalence rates are age-standardized to the Mexican Pima population and presented with 95% confidence intervals. Mex Pimas Non Pima Mex AZ Pimas NIDDM% 6,4 (2,2,10,6) 3,4 (3,3,6,4) 38,2 (33,8,42,6) IGT 6,4 (2,2,10,6) 8,2 (3,8,12,5) 10,5 (8,4,12,6) The mean BMI in the Mexican Pimas was 24.9+/-3.9 (SD). This was similar to the non-Pima Mexicans (25.7+/-4.6), and lower than in the Arizona Pimas (34.2+/-8.2). The prevalence of NIDDM in the Mexican Pimas was one-sixth of that in the Arizona Pimas, whereas the rate in the Mexican Pimas was higher, but not significantly so, than in non-Pimas. These results indicate the importance of environmental factors in preventing the development of both NIDDM and obesity in populations who presumably carry a genetic susceptibility to these conditions.